Buffer overflows are perhaps the most notorious and widely publicized attacks. These are complex attacks that exploit the fundamental hardware and software capabilities of a system.

For those who are not software developers, a few concepts need explanation. First, it is useful to understand what a buffer is. The computer system has a pool of Random Access Memory (RAM) organized into small chunks by the operating system that runs applications. In order to share this memory among the operating system's many processes and applications, a special memory manager coordinates which chunks of the RAM pool are in use and which are available to run an application. When an application is first run, memory is allocated for the application and all of its functions and variables.

As the application runs, more memory can be allocated for new variables and de-allocated when no longer in use. A buffer is a chunk (or several chunks) of memory used to store a variable. Different buffers can and often do exist side-by-side in memory. A buffer that holds a variable can exist next to a piece of memory that holds a function or another application. For example, when you enter your username at a prompt or window, the program has declared a buffer, in which the characters of the name are stored.

A buffer overflow occurs when a buffer is too small to accommodate the amount of data provided. The data that does not fit into the buffer will overwrite the next chunks of memory. Herein lies the danger of buffer overflows. The memory that is overwritten with the extra data can be another variable in the running application, a variable for another application, or the application's stack. The stack contains application-specific information, such as the physical locations of the application's functions and variables.

This alters the path that the application would normally follow, causing it to use bad data, crash, or execute new functionality. The execution of new functionality is usually the goal of a buffer overflow exploit—whether to provide access to the system or to modify its settings.

When its owner accesses the overwritten memory next, the new data might be invalid, and the application can crash or function improperly. Buffer overflows are exploited by crafting the overflow data into something useful that the machine can understand. This could mean executing another program, causing harm to the system or stealing information.

To better demonstrate this concept, consider the postal machine that scans letters for their destination city. Assume envelope A is destined for New York, NY, and envelope B is destined for Boston, MA. The ZIP code of envelope B has special wet ink that "overflows," overwriting the ZIP code of New York on envelope A with that of Boston when the envelopes are automatically stacked at the postal facility. The postal machine scans envelope A and reads the overwritten ZIP code. The letter is then routed to Boston.

Buffer overflows are often more malicious than mere accidents, so let us assume that a valuable sum of cash is destined for a postal box in New York. The thief, or attacker, knows the exact location of the letter and creates an envelope with runny ink that overwrites the ZIP code on the envelope with the ZIP code for Boston when they are stacked. The attacker rents the Boston postal box with the same number as the original destination in New York and proceeds to steal the money.

A postal machine is only capable of recognizing ZIP codes, no matter from where they come. A computer is capable of executing instructions, no matter from where they come. Therefore, a buffer overflow attack that overwrites the original instructions of an application with new instructions can cause the computer to execute anything an attacker wishes.